1. Field of the Invention
The present invention relates to an optical waveguide board and an optical module and more particularly to the optical waveguide board and the optical module to be used for multi-mode optical transmission.
The present invention claims priority of Japanese Patent Application No. 2001-337044 filed on Nov. 1, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
Recently optical communication technology using light as an information transmission medium has become widely used. In such optical communication technology, in order to transmit signal light, an optical waveguide board having an optical waveguide formed on a substrate is employed in which signal light is transmitted along the optical waveguide. The optical waveguide is configured so that its core layer having a high refractive index is surrounded by its clad layer having a low refractive index. Signal light having been incident on one end of the optical waveguide at an angle larger than a critical angle is totally reflected at a boundary between the core layer and clad layer such that the signal light is confined within the core layer and propagates along a direction of a length of the core layer and is then output from another end of the optical waveguide. To perform such an optical transmission, a multi-mode optical transmission system is employed which is adapted to transmit light signal made up of multi-order light.
While such a multi-mode transmission is carried out, signal light travels while in being repeatedly totally reflected at the boundary between the core layer and clad layer at a large angle therefor the number of times the reflection is repeated per unit distance is increased compared with a case of a low-order mode transmission and, as a result, transmission speed is limited causing an increase in transmission loss and a decreased output of outgoing light. To solve this problem, an optical waveguide board (optical signal transmission system) is disclosed in, for example, Japanese Patent Application Laid-open No. 2000-39531. The disclosed optical waveguide board is configured so that, by forming a curved convex surface portion at an end face of an optical waveguide on which signal light is incident, as shown in FIG. 19A, signal light 51 is converted into parallel light by lens effects of the curved convex surface portion and is made incident into a core layer 52. By constructing the optical waveguide as above, since only zero-order-mode signal light can be output as parallel light, the intensity of the outgoing light becomes high, which enables a decrease in light output to be avoided.
However, the above disclosed conventional optical waveguide board has a problem in that, if the parallel light being incident on a incident end of the optical waveguide is inclined in its incident direction, since high-order-mode signal light corresponding to this inclination is transmitted, transmission characteristics of the signal light are changed. That is, as shown in FIG. 19B, if the parallel signal light 51 is incident on an incident end of the core layer 52 with an inclination of an angle G, high-order-mode signal light corresponding to the inclination of the angle G is transmitted, and the intensity of the output zero-order-mode signal light is low. When optical modulating or optical switching processing is performed, zero-order-mode signal light is used. However, as described above, since the intensity of the zero-order-mode signal light is low, variation in transmission characteristics of signal light becomes unavoidable.